Refractometers work with water and sugar. You have water and alcohol at the end of fermentation, which will not give you accurate readings.

I believe there is a graph somewhere where you can look up the value you read and get the true value.

The floating hydrometers works as expected, but the refractometers are convenient. You just have to know when to use them 🙂

Refractometers are one of the most misunderstood tools in home brewing. The two biggest mistakes are using the sg side of a dual brix/sg scale (the conversion is non-linear, and the scale will likely become increasingly off between the two the higher the gravity), and using one like a hydrometer, ie simply subtracting final from original gravity. Instead, only use the brix side of the scale, and use a calculator such as this one to determine the actual specific gravity/abv once fermentation begins: https://www.brewersfriend.com/refractometer-calculator/ (don't worry too much about the Wort Correction Factor.) Do that, and your refractometer will be your best friend.